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In a PNAS paper, Smith et al. (1) demonstrate spontaneous self-assembly of a DNA analog from a solution of mononucleotide triphosphates, exhibiting concurrently a high degree of Watson–Crick base pair selectivity with complementarity-dependent partitioning of nucleotides, a stacking order corresponding very closely to B-DNA geometry and a mesophase ordering similar to the one observed for much longer segments of DNA. This biopolymer “liquid crystal autocatalysis,” enabled by the three levels of soft, hierarchical assembly and exhibiting key DNA structural elements, can eventually lead to molecular selection, as well as catalytically guide and facilitate the final ligation of prebiotic DNA analogs into structurally stiff DNA.

Aqueous solutions of long- as well as short-fragment DNA exhibit a sequence of ordered liquid-crystalline (LC) mesophases at DNA concentrations consistent with in vitro conditions (2). This spontaneous DNA ordering, relevant from the fundamental biophysical point of view (3, 4), is also indispensible for DNA packing within bacteriophage capsids at high osmotic pressures (4), in eukaryotic sperm cells where DNA is packaged by a variety of basic (i.e., positively charged) polypeptide chains (5), as well as in the spatial organization of chromatin inside the eukaryotic nucleus (6). Starting from the high-density end, the sequence of ordered phases in DNA solutions starts with an orthorhombic (OR) crystalline phase (7) with DNA in the A-form conformation, corresponding to its pronounced dehydration (8). Shorter 50-nm nucleosomal DNA fragments are in the OR phase at somewhat higher density but do not show the A-form fingerprint (9). At lower densities, long DNA enters the line hexatic (LH) LC phase (10), characterized by long-range orientational and bond orientational orders but with short-range positional order (11). The OR→LH transition is characterized by a discontinuous DNA density change and the emergence of a sixfold symmetric scattering intensity profile observed for oriented samples (7 …

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